1. Field of the Invention
The invention relates to a method for detecting hidden objects via electromagnetic millimeter waves, particularly of objects carried by persons.
2. Description of the Background Art
Methods for screening persons, such as flight passengers, for suspicious objects, for example, weapons or explosives, are known in which the persons are scanned by millimeter waves. Methods and devices of this type are described in the International patent applications WO 2006/105977 A1, which corresponds to U.S. Pat. No. 7,583,221, and WO 2007/028472 A1, which corresponds to U.S. Pat. No. 7,609,196.
Depth profiles are to be taken close to a millimeter wave scanner, in the so-called near field, to detect objects carried by persons on their body. With the aid of these depth profiles, the reflecting signals from weakly reflecting objects can be detected, which are normally hidden by the signals of more strongly reflecting objects in other depth layers. Examples of weakly reflecting objects, whose signals are hidden by strongly reflecting objects, are plastic bonded explosives, which have a low millimeter wave reflection, on human skin, which reflects millimeter waves strongly. Likewise, the signals of weakly reflecting plastic mines are hidden, which are located under the highly reflective surface of the earth.
Typically, the depth resolution by means of millimeter waves occurs by the transmission and evaluation of extremely short pulses or by frequency modulation as in FMCW radar. A possible special form of FMCW radar is the so-called stepped-FMCW radar, in which discrete frequencies are used, which are counted back with the aid of Fourier transformation to pulses within the time domain. All of these methods require a very wide bandwidth, if a depth resolution of a few centimeters is required. Thus, a bandwidth of 10 GHz is needed for a depth resolution of 1.5 cm. When the stepped-FMCW radar is used, a relatively small increment of the frequencies is required to avoid phantom targets. Thus, a maximum increment of 100 MHz is permissible for an unambiguous range of 1.5 m.
These requirements are technically very difficult and very expensive to realize. It is difficult, furthermore, to obtain approvals from the authorities for the use of sufficiently wide bandwidths. The effort was therefore made to use modeling methods to obtain the required resolution at a smaller bandwidth. These methods are very costly and cannot be used for real-time systems.